Intermission - Sold mortage?

My nephew and his wife have bought a house and some land in a very nice area. It's out in the country where it's quiet, and the house is a double wide mobile home that's absolutely huge. Their master bath is as large as my living room!

However, they have recently received papers that their mortgage lenders have changed. The original lender sold the account to another lender before their first payment was even made. This was a bit disconcerting for my nephew, but on the upside, his payments are actually lower now than before.

So, tell me, does this happen often? Does it happen to everyone, or just to folks like us who are not in the upper echelon of income and notoriety? What if it changes hands again? Will the payments go up? It's supposed to be locked in at a certain interest level so they would know from month to month and year to year what to budget for their payments. Of course, if they were among the rich and famous, they could just pay the whole thing off and not have to worry about any of that!